<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* .box{
            width: 200px;
            height: 200px;
            margin: 100px auto;
            background-color: #ccc;
            border-radius: 10px;
            border-top-left-radius: 50px;
            border-bottom-right-radius: 50px;
        } */

        .box{
            width: 0px;
            height: 0px;
            margin: 100px auto;
            border-left: 100px solid transparent;
            border-top: 0px solid transparent;
            border-right: 100px solid transparent;
            border-bottom: 100px solid red;
        }

    </style>
</head>
<body>
    <div class="box">

    </div>  
</body>
</html>